Implementation for GetCtfToETWMappings. (#1899)

* Implementation for GetCtfToETWMappings.

This allows 3rd party parsers to register for known events and process
trace file properly.

Current parsers (like ClrTraceEventParser) are left unoutched, because
these are initialized lazily (i.e.
https://github.com/microsoft/perfview/blob/680ed266e842e49a3c5cd0af89eb7e4f9645b785/src/TraceEvent/TraceEvent.cs#L170).

* Resolve first feedback.

* Resolve second feedback.

* Better lookup for getting mapping.

Author: cincuranet